The present invention relates to a switching unit used in pocketable telephones, car-borne telephones, remote controllers, handy terminals and the like or, more particularly, to an illuminable push button switching unit with a covering member having self-illuminated key tops containing a luminescent material such as a phosphorescent pigment and the like mounted on a circuit board.
The push button switching unit in pocketable telephones and the like is constituted of an encased assembly of a circuit board and a covering member mounted thereon. Generally speaking, such a covering member has, on the upper surface, a plurality of key top parts each of which has an indicia such as letters, signs, symbols and the like formed on the upper surface and a movable contact point on the lower surface thereof to correspond to the respective key tops. When the key top is pressed down with the fingertip of an operator, the movable contact point on the lower surface of the key top is brought into contact with the fixed contact points on the circuit board either directly or indirectly with intervention of a film having a printed electroconductive pattern so as to close the electric circuit. It is conventional that, in a push button switching unit of this type, a large number or, for example, from around 10 to 15 pieces of light sources such as LEDs and the like are mounted in the lower part of the covering member to internally illuminate the indicias in order to increase recognizability thereof.
In the conventional internally illuminable push button switching units, however, it is a difficult matter to evenly illuminate the indicias on the top surfaces of all of the key tops and unevenness is unavoidable in the illumination and brightness of the illuminated indicias. In addition, a problem is encountered in the large electric power consumption because a large number of LEDs are simultaneous lighted and, since illumination of the indicias is limited only to the moment when the switching unit is under operation, difficulties are encountered in the recognition of the indicias when the switching unit must be handled in a dark place or in night.
The latter problem can be solved, needless to say, by lighting the LEDs constantly throughout but this way causes another problem that the power consumption is further increased resulting in shortening of the operating duration of the instrument such as pocketable telephones or requirement for a battery pack of a large capacity with an unavoidable increase in the weight of the instrument.